\nCO2 is a colorless, odorless gas, approximately 1.5 times heavier than air. It is one of the most common indoor pollutants, primarily because the source of this gas is mainly human activity.
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Measurement of CO2 concentration
\nThe concentration of carbon dioxide is expressed in ppm (parts per million). For example, a measured value of 1,500 ppm means 1,500 particles in the air per million others. Sometimes it is also expressed in %.
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Recommended values
\nThe so-called Pettenkofer criterion (Pettenkofer, 1858), which sets the maximum concentration of CO2 in indoor spaces at which a person still feels comfortable, is 1,000 ppm (0.1%). This value has also been adopted by the international organization ASHRAE – a professional society with over 50,000 members focused on building technology and building systems with an emphasis on heating, ventilation, and air conditioning. It supports technical education, participates in certifications and standardization, publishes the ASHRAE Journal, and releases summaries of research reports and information about conferences.
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\nA concentration value of 1,000 ppm of carbon dioxide in the interior is therefore the upper limit for quality indoor air in the USA. At higher concentrations, fatigue and lack of concentration usually occur in individuals present, and in worse cases, you may experience headaches or dizziness. Air with a carbon dioxide concentration above 1,500 ppm (= threshold value in the Czech Republic) in the interior is considered stale air.
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\nAlthough the safe threshold for carbon dioxide concentration that does not pose serious health risks to humans is 5,000 ppm, it is important to keep CO2 concentration below one thousand ppm.
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Specific measurements
\nTo determine the actual values of CO2 concentration in places where people spend most of their time, our company placed air quality sensors in two kindergarten classrooms, one elementary school classroom, and two offices.
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Kindergarten
\nMeasurements were taken in two classrooms of the same size, 8 x 7 m, where a group of about 24 children and teachers were present. Because these were relatively large rooms where children, who produce lower amounts of CO2, spent time, the rooms were well-ventilated, and the children often spent time outside these rooms (morning walks, lunch, and nap after lunch), the CO2 value rarely exceeded the threshold of 2,000 ppm. Nevertheless, values often exceeded 1,500 ppm – the mandated value at which ventilation is required in the Czech Republic.
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Figure 1: Measurements in the kindergarten (large drop = weekend, smaller drops = night, CO2 decreases during the day = walks, lunch,…)
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Elementary School
\nMeasurements at the elementary school were conducted in the 1st grade (i.e., children aged 6-7 years) in a classroom measuring 8.5 x 9 m. The classroom was therefore larger than in the kindergarten, with about 26 children present, and only small breaks when this classroom could not be ventilated to a tolerable CO2 concentration. Values during the day typically exceeded a concentration of 2,000 ppm and several times reached above 3,000 ppm. This fact may also explain the frequent fatigue and inattentiveness of children during lessons.
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Figure 2: Measurements in the school (large drop = weekend, smaller drops = night, CO2 decreases during the day = walks, lunch,…)
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Office
\nThe measured values in the office were the lowest of all measurements. This fact is due to the ventilation system implemented in the company. In addition, it is also a relatively large room (6 x 7.5 m) with high ceilings and a low number of workers in the room.
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